1. Technical Field of the Invention
The present invention relates in general to optical devices, and more particularly to distributed Bragg reflectors and their fabrication.
2. Description of Related Art
A distributed Bragg reflector (DBR) is a periodic grating that, in a semiconductor system, can be monolithically formed on a wafer from alternating layers of differing index of refraction. DBRs have applications in various optic devices, in part because DBRs can achieve a high reflectivity in a relatively compact space. Further, DBRs can be tested immediately after fabrication on a wafer, unlike a crystalline reflector that must be cleaved prior to testing. Examples of devices that have incorporated DBRs include tunable optic filters, tunable detectors, and surface emitting lasers including vertical cavity surface emitting lasers (VCSEL).
The reflectivity of a DBR is a function of both its geometry and the relative difference between the index of refraction of the layers. The relative difference in the index of refraction of two materials is referred to as the index contrast. Generally, the reflectivity increases as the index contrast between layers increases and as the number of layers of the DBR increases. Also, the stop band width of the DBR increases as the index contrast increases.
A DBR can be formed from layers of semiconductor or dielectric materials layered together using known semiconductor fabrication techniques. For example, indium gallium arsenide phosphide (InGaAsP) can be layered together with indium phosphide (InP) (InGaAsP/InP DBR). Because the index of contrast between InGaAsP and InP is relatively small, on the order of 0.18, the number of layers needed to achieve a given reflectivity is high. Also, the stop band width is relatively small. In another example, silicon dioxide (SiO2) and titanium dioxide (TiO2) can be layered together (SiO2/TiO2 DBR). SiO2 and TiO2 have a high index of contrast, on the order of 0.77, so relatively fewer layers are needed to achieve the same reflectivity. In comparison to a InGaAsP/InP DBR, a SiO2/TiO2 DBR can be more compact while achieving the same reflectivity. This combination also has a broader stopband width than an InGaAsP and InP DBR.
In a final example, an air/semiconductor DBR can be formed where layers of a semiconductor material, such as InP, are spaced apart by air gaps. Air and InP have a high index contrast of 2.2. An air/semiconductor DBR can achieve a high reflectivity with a relatively small number of layers over a broad stopband width because the index contrast between most semiconductor materials and air is large. In comparison with a InGaAsP/InP DBR or a SiO2/TiO2 DBR, the air/semiconductor DBR can be the most compact at a given reflectivity.
Air/semiconductor DBRs are fabricated by growing or depositing two different epitaxial films onto a substrate. The film materials are chosen to have a high etch selectivity, allowing one film to be substantially etched while leaving the other substantially intact. The film layers are masked and etched into one or more mesa formations. A DBR will be constructed from each mesa. A selective etch is used to remove or undercut portions of one material, thus creating air gaps between cantilevered layers of remaining material.
Constructing an air/semiconductor DBR is a difficult process, because the air gaps are unstable and can easily collapse both during and after the fabrication process. Residual stresses in the remaining material, resulting from the growth or deposition process, can cause the remaining material to collapse and close off the air gaps. Thus, prior art DBR fabrication techniques have sought to carefully control the film deposition or growth procedure to minimize the residual stresses. Also, when the etchant is rinsed from the air gaps, the surface tension of the fluid rinse leaving the air gaps tends to pull the remaining material together and collapse the air gaps. Prior art DBR fabrication techniques have tried to overcome this difficulty by utilizing fluid rinses with low surface tension to minimize the tendency of the air gaps to collapse. Other prior art DBR fabrication techniques have used critical point freeze drying or sublimation drying, where the rinse is sublimated or quickly evaporated by dropping the ambient pressure or temperature, to prevent the surface tension of the rinse from collapsing the air gaps. These methods have been mostly effective, but there still exists a possibility that the air gaps will collapse.
Thus, there is a need for an improved air/semiconductor distributed Bragg reflector and method of fabricating the same that better prevents collapse of the air gaps.